The invention relates to a multi-component gas analyzer having cassette-type light path system, and more particularly, relates to a unified and systemized gas analyzer that can analyze multi-component gases at the same time without any movable parts.
The health problems, affected by modern living environment of human being and caused by the air pollution resulted from industrialization and dense population, have called wide attention in all walk of life. There are problems of how to analyze and judge the air quality, to what extent of the air is polluted, what kinds of air components are noxious to the health of human being, what the percentage of noxious gas components contained in the air. These problems depend on reliable and precise apparatus to analyze and to measure. Owing to the regulations and air pollution policies imposed by the government at all levels, relatively high demands are requested on the indoor and outdoor air quality as well as on the single and multi-component gas analyzers for analyzing and measuring the exhaust gases of automobiles and motorcycles. In general, the mandate gas components needed to be analyzed and measured are carbon dioxide (CO2), hydrocarbon (HC), and carbon mono-oxide (CO) etc. There are many ways of analyzing and measuring the gas components among the many gas analyzers, and currently, a non-dispersive infrared (NDIR) spectrometry is considered to be the most popular one.
NDIR is an optical absorption method based on the spectral selected principle. Basically, each kind of gas has one or multiple different Infrared (IR) absorption characteristics. In other word, the IR absorbance for each gas having a specific wave-length relates directly to the concentration of the gas. This kind of IR absorbance principle for a gas is called Beer-Lambert Law. As shown in FIG. 1, the IR absorbance for a gas is proportional to the concentration and the length of light path of the gas and is shown by the following formula
A=Kxc3x97Cxc3x97L 
Where
A=IR absorbance that takes the logarithm of the ratio of the light intensity of the original incident light to the light intensity after the light is absorbed;
K=the IR absorption coefficient of the gas;
C=the concentration of the gas;
L=the length of light path through which the gas absorbs the IR.
As shown in FIG. 1, while ideally, the absorbance of the gas to IR is linearly proportional to the concentration of the gas, in reality, there is always a discrepancy, thereby, a nonlinear relationship exist between them. Moreover, the higher the optical density, the higher the extent of the discrepancy will be. The optical density denotes the product (Cxc3x97L) of the gas concentration C and the length of the light path L. To lower the discrepancy so as to improve the measuring accuracy of the system, the designer of the system needs to select an optimum optical density. In other word, a gas having relatively high concentration or high absorbance needs to select a gas analyzer with relatively short light path, and vice versa. Consequently, the design of a gas analyzer that is capable of measuring multi-gas requires that the gas analyzer can select to use a multiple of lengths of light path so as to extend the measurable dynamic range of the apparatus.
Moreover, the frequency range of a specific gas is a constant value, thereby, a specific filter corresponding to the frequency range is required in order to filter and select an IR of a single specific frequency. Thereafter, a sensor is used to sense the variation of the light intensity of the IR. There is also a design option to have a detector combining sensors with filters.
The first prior art quoted by the invention is a multi-channel gas sample chamber disclosed by the U.S. Pat. No. 5,222,389. As shown in FIG. 2, the characteristic of the first prior art is that a plurality of detectors 206, 208, and 210 are provided respectively at their detector ports 212, 214, and 216 on the circumference 202 of a long cylindrical hollow light tube. In addition, a detector 218 is also provided at the end of the light exit. These detectors are used for measuring the multi-gas in response to the above-mentioned principle, that is, a gas having relatively high absorbance to IR needs to select a gas analyzer with relatively short light path. On the contrary, a gas having relatively low absorbance to IR needs to select a gas analyzer with relatively long light path.
However, the first prior art is unable to substantially attain the expected effect for the following reasons:
1. The IR, denoted by arrow heads as shown in FIG. 2, received at the detector 206208 and 210 that are provided on the circumference 202 of the light tube are incident lights at skew angles rather than at right angles. Since optimum effect of gas measuring can be obtained if the incident lights are at right angles, thereby, the incident IR lights at skew angle will affect the accuracy of the output frequency of the filters in the detectors.
2. Since the incident lights transmitted into the detectors are at skew angles, the intensity of the incident lights is attenuated, consequently, the output signal/noise ratio is lower.
The second prior art quoted by the invention is a multiple component gas analyzer disclosed by the U.S. Pat. No. 4,914,719. As shown in FIG. 3, the multiple component gas analyzer includes an IR source 300 that can generate light beam; an optical absorption chamber 302; three beam splitter 304, 306, 308; four detectors 310, 312, 314, 316; four filters 320, 322, 324, 326. The light beam transmits through the optical absorption chamber 302, and is then guided into the splitters 320, 322, 324, 326 to become split light beams that are reflected by the splitters 320, 322, 324, 326 and are deflected to the detectors 310, 312, 314, 316. One of the light beams that transmits directly all the way through the series of splitters 320322, 324, 326 hits into the detector 316 through the filter 326. Each of the filters 320, 322, 324, and 326 corresponds respectively to a specific distinctive frequency range.
However, the second prior art can not substantially attain the effect of extending the measurable dynamic range of the apparatus. Since the length of the optical absorption chamber 302 is the only length that is available, there are no multiple lengths of light path to be selected in order to match the different gas concentration. Here are some of the examples showing the resulting situations.
1. According to the Beer-Lambert Law, the absorption intensity of the gas to the IR will be rather high if a relatively long light path is employed by a relatively high concentration of gas. This will result in the fact that the absorption of the gas to the IR is apt to attain saturation which makes the light intensity measured by the sensor rather weak.
2. On the other hand, according to the Beer-Lambert Law, the absorption intensity of the gas to the IR will be rather low if a relatively short light path is employed by a relatively low concentration of gas. This will result in the fact that the absorption of the gas to the IR is little which makes the variation of the light intensity measured by the sensor is rather limited. Consequently, the detecting ability of the analyzer is relatively low,
3. The above-mentioned extreme cases will result in the fact that the concentration range that can be measured by the gas analyzer is limited. In other word, a gas analyzer having relatively short light path and is adequate for measuring gases having relatively high concentration will make the range of measuring gases having relatively low concentration become narrow. On the contrary, a gas analyzer having relatively long light path and is adequate for measuring gases having relatively low concentration will make the range of measuring gases having relatively high concentration become narrow too.
The third prior art quoted by the invention is a gas analyzer disclosed by the U.S. Pat. No. 5,773,828. As shown in FIG. 4(a), one of the embodiments of the third prior art is a two-component measuring gas analyzer. The gas analyzer has a NOx gas measuring cell 403 having a length L2 of around 60 mm that is connected through a communication part 421 to a CO2 gas measuring cell 407 having a length L1 of around 1 mm. A filter 440, which is provided near the light source 401 and used in a apparatus for transmitting and reflecting the light, is employed to spectrally diffracting a specific wave length of IR. An IR A, originated from the light source and with its specific wave length xcex2, transmits through a band pass filter 440 to become A2, and with its another wave length xcex1, reflects from the band pass filter 440 to become A1. A NOx detector 405 is provided at an end of the gas measuring cell 403 while a CO2 detector 409 is provided at an end of the gas measuring cell 407. By the use of this apparatus, two different gases NOx and CO2 can be measured by the two mutually connected gas measuring cells 403 and 407 which have different lengths L2 and L1 respectively, through a communication part 421.
FIG. 4(b) is a gas analyzer of another embodiment of the third prior art disclosed by the same U.S. Pat. No. 5,773,828. As shown in FIG. 4(b), another filter 444 provided between the gas measuring cell 403 and the NOx detector 405 is also employed to spectrally diffracting another specific wave length of IR. The IR A2, transmitted through the band pass filter 440 and with its specific wave length xcex4, further transmits through a band pass filter 444 to become A4, and with its another wave length xcex3, reflects from the band pass filter 444 to become A3. A NOx detector 405 is provided at an end of the gas measuring cell 403 while a CO2 detector 412 is provided at an end of the gas measuring cell 423. By the use of this apparatus, two different gases NOx and CO2 can be measured by the two mutually connected gas measuring cells 403 and 423, which have different lengths L2 and L2+L3 respectively, through a communication part 425.
FIG. 4(c) is a gas analyzer of one further embodiment of the third prior art disclosed by the same U.S. Pat. No. 5,773,828. As shown in FIG. 4(c), another gas measuring cell 477 having length L4 is added to the gas measuring cell 403 as shown in FIG. 4(a) in order to add another function for measuring CO gas component. A band pass filter 442 and a CO gas detector 441 are provided at an end of the gas measuring cell 477.
Although the third prior art as mentioned above can match appropriate light path length in accordance with the concentration of the gas components intended to measure, there are still some points of imperfections that are listed as follows
1. The extension of the light path length and the flowing of the gases lack of systematic scheme, thereby, the extension of the gas measuring cells and the set-up of the communication parts grow wild with a lot of branches. Consequently, not only that the apparatus occupies a lot of spaces but the gas flowing through the apparatus is not uniformly distributed also.
2. The proportion of beam-splitting is a fixed one and is unable to be flexibly adjusted, as a result, the light intensity is attenuated after the light path is extended.
In the light of the disadvantages of the above-mentioned prior arts, one of objectives of the invention is to provide a cassette-type light path system with systematic scheme. The cassette-type light path system can systematically stack up the cassette-type of light absorption chambers to match with appropriate length of light path in accordance with the concentration of the gases intended to measure. This cassette-type light path system of the invention, being not only compact in structural design but also neat, tidy, and good order in shape, is able to save a lot of space, to become portable, and easy to manufacture, thereby, can lower the manufacturing cost.
Another objective of the invention is to provide an integrated light-absorption chamber of cassette-type guided-flow gas. By the use of a plurality of vent hole sets provided on both sides of each of the stacked cassettes, the gases to be analyzed can pass through the cassettes smoothly.
One other objective of the invention is provide a semi-hollow beam splitter in an optical folding system that can adjust the beam splitting proportion effectively. The transmitting beam component in the conventional technology is replaced by a direct method in the invention to have the IR light pass directly through the hollow portion of the beam splitter. In this way, the rate of attenuation of the light intensity is greatly reduced. The proportion of beam splitting is achieved by adjusting the proportion of the hollow portion and the reflective portion. Consequently, the light intensity after beam splitting is maintained in a rational range through adjusting each of the cassette type optical absorption chamber by the adjustment of the beam-splitting proportion.
In order to achieve the above-mentioned objectives, the invention provides a multi-component gas analyzer having a light source system, a cassette-type light path system, an optical folding system, and an infrared detecting system. The light source V system has an infrared light source and a reflector for providing a parallel infrared light beam. The cassette-type light path system has a cassette, an optical absorption chamber, and a set of concave mirrors. A vent hole set having a plurality of vent holes enclosed by an O-ring for sealing the gas between the stacked-up cassettes is provided on the surfaces of the cassette and is disposed in staggered pattern so as to uniformly distribute the gas introduced. An inlet and an outlet for inputting and outputting the parallel infrared light beam are also provided at the cassette. The optical absorption chamber having a highly reflective coating film coated on the inner wall surface thereof is employed for introducing the gases to be tested and providing the space for contact between the gases and the parallel infrared light beam. A set of reflective concave mirrors disposed in staggered pattern on the two opposite sides therein is provided in the optical absorption chamber for guiding the parallel infrared light beam and extending the light path. The optical folding system includes a beam splitter for splitting and distributing the infrared light beam into each of the cassette, and a deflector for guiding infrared light beam to transmit into the next cassette. Finally, the infrared detecting system includes a reference detector and a measuring detector wherein each detector has a filter and a sensor. The filter in the reference detector is distinct from the one in the measuring detector for sensing infrared in closed but distinct frequencies while the sensors in both detectors are exactly the same.
Regarding the invention""s object, advantages and characteristics as above-mentioned or others can be better understood by the followingxe2x80x9cdetailed description of the preferred embodimentsxe2x80x9d.